Do Potential Methylation Rates Reflect Accumulated Methyl Mercury in Contaminated Sediments?

Abstract

Relationships between the short-term mono-methyl mercury (MeHg) production, determined as the specific, potential methylation rate constant Km (day(-1)) after 48 h of incubation with isotope-enriched 201Hg(II) at 23 degrees C, and the long-term accumulation of ambient MeHg, were investigated in contaminated sediments. The sediments covered a range of environments from small freshwater lakes to large brackish water estuaries and differed with respect to source and concentration of Hg, salinity, primary productivity, quantity and quality of organic matter, and temperature climate. Significant (p < 0.001), positive relationships were observed between Km (day(-1)) and the concentration of MeHg normalized to total Hg (%MeHg) for surface sediments (0-10, 0-15, and in one case 0-20 cm) across all environments, and across subsets of organic and minerogenic freshwaters. This suggests that the methylation process (MeHg production) overruled demethylation and net transport processes in the surface sediments. The lack of a relationship between Km and %MeHg in two brackish water sediment depth profiles (0-100 cm) indicates that demethylation and the net effect of input-output are relatively more important at greater depths. Differences in the primary production and subsequent availability of easily degradable organic matter (serving as electron donor for methylating bacteria) was indicated to be the most important factor behind observed differences in %MeHg and Km among sites. In contrast, concentrations of sulfate were not correlated to Km, %MeHg, or absolute concentrations of MeHg. We conclude that total concentrations of Hg are of importance for the long-term accumulation of MeHg, and that %MeHg in surface sediments can be used as a proxy for the rate of methylation, across a range of sites from different environments.

Monomethylmercury behaviour in sediments collected from a mercury-contaminated lagoon

“…This greater anoxic character of the sediments seems to be essential to the occurrence of Hg methylation. In fact it is in this oxic/anoxic layer of the sediments that it is expected the highest activity of sulphate-reducing bacteria, which are the most important methylators for Hg, in a similar way as that observed by Drott et al [24] for sediments collected in several contaminated places. In this layer, bacteria are able to compete with respect to electron acceptors (such as O 2 , Fe(III)) and have access to energy-rich organic matter (electron donor).…”

“…In some studies, for example, Benoit et al [21] and Drott et al [24], the existence of a positive correlation between total Hg and MeHg is reported, suggesting that both Hg species (inorganic Hg and MeHg) were co-deposited or the in situ MeHg production was a function of total Hg concentration. In Largo do Laranjo, on the contrary, there were places with total Hg concentrations higher than those determined in site 1 and where MeHg was not detected.…”

Influence of a wastewater treatment plant on mercury contamination and sediment characteristics in Vidy Bay (Lake Geneva, Switzerland)

“…Drott et al 2008). In the present study, since the MMHg PW / THg PW ratio was generally higher in the 0-1.5 cm layer than in deeper layers, the main production of MMHg likely occurs in the surface of sediments.…”

Recent advances in the study of mercury methylation in aquatic systems

“…These short-term measurements of potential rates of methylation and demethylation have been found to be unrelated to gross measures of long-term MeHg accumulation (Drott et al 2008;Johnson et al 2016). This suggests that these snapshot measurements may not be valuable in determining long-term process measurements.…”

“…This suggests that these snapshot measurements may not be valuable in determining long-term process measurements. However, significant positive correlations have been found between K meth and %MeHg in many environments (Drott et al 2008), and %MeHg values in boreal wetlands tracked the ratio of K meth to K demeth (Tjerngren et al 2012a), suggesting that if both rates are measured, steady state net MeHg production rates can be estimated. Very recently, measurements of potential rates have been replaced with measurements of the genetic potential of the microbial community (e.g., Gionfriddo et al 2016) due to the recent discovery of a gene cluster responsible for mercury methylation (see below), and developments in this area will be useful in addressing many of the questions previously addressed using the measurements of potential rates.…”